Impact of ZnO NPs on photosynthesis in rice leaves plants grown in saline-sodic soil

Saline-sodic stress restricts the absorption of zinc by rice, consequently impacting the photosynthesis process of rice plants. In this experiment, Landrace 9 was selected as the test material and the potting method was employed to investigate the influence of ZnO nanoparticles (ZnO NPs) on zinc absorption and chlorophyll fluorescence in rice grown in saline-sodic land. The research findings demonstrate that the application of ZnO NPs proves to be more advantageous for the growth of rice in saline-sodic soil. Notably, the application of ZnO NPs significantly decreases the levels of Na+ and MDA in rice leaves in saline-sodic soil, while increasing the levels of K+ and Zn2+. Additionally, ZnO NPs enhances the content of chloroplast pigments, specific energy flux, quantum yield, and the performance of active PSII reaction center (PIABS) in rice leaves under saline-sodic stress. Furthermore, the relative variable fluorescence (WK and VJ) and quantum energy dissipation rate (φDo) of rice are also reduced. Therefore, the addition of ZnO NPs enhances the transfer of electrons and energy within the rice photosystem when subjected to saline-sodic stress. This promotes photosynthesis in rice plants growing in saline-sodic land, increasing their resistance to saline-sodic stress and ultimately facilitating their growth and development.


Experimental site
The pot experiment was conducted at Jilin Agricultural University in Changchun, Jilin Province, China (E 125°21 ′, N 43°52 ′) from April 2022 to October 2022, the specific location is shown in Fig. 1.The location has a temperate continental sub-humid monsoon climate with a sunshine duration of 2685 h.The accumulated temperature during the experiment ranged from 2840.1 to 3040 ℃ after reaching a minimum of 10 ℃.The frost-free period lasted for 145-150 days.The saline-sodic soil used in this study was obtained from Sheli Town, Da'an City, Jilin Province, China (N 45°35′58″-N 45°36′28″, E 123°50′27″-123°51′31″).The soil samples were collected, air-dried, and sieved through a 2 mm sieve.The table below (Table 1) provides the basic physical and chemical properties of the soil before the test.According to the World Soil Resources Reference Basis 29 .

Experimental design
The rice variety used in this experiment is Changbai 9, which is one of the excellent varieties cultivated in the saline-sodic paddy soil in northeast China.The pot planting method was employed, following a random block design.On April 12, 2022, rice seeds were sown in a greenhouse.Rice seedlings with similar growth were selected and transplanted into pots (upper diameter 30.50 cm, lower diameter 20.40 cm, height 26.20 cm, with 15.00 kg of soil in each pot) on May 21, 2022, at the three-leaf stage.Each pot contained three points, with three plants in each hole.There were a total of 25 pots in each treatment, and a total of three treatments.In this experiment, contrast (CT), ZnSO 4 (Z), and ZnO NPs (nZ) were used.ZnSO 4 heptahydrate at a concentration of 45 kg ha −1 was identified as the optimal concentration based on previous experiments.The ZnO NPs used in the study was sourced from Shanghai Chaowei Nano Technology Co., LTD, with an average particle size of 50 nm and a purity of 99.9%.The characterization structure of nanometer zinc oxide is shown in Fig. 2. The application amount of ZnO NPs was 30 kg ha -1 .Both zinc sources were mixed with 5 cm of surface soil in the basin during transplanting.Detailed application amounts of ZnSO 4 and ZnO NPs are shown in Table 2.The fertilizer dosage for each pot was calculated based on the field's recommended application rate (N 250 kg ha −1 , P 100 kg ha −1 , K 120 kg ha −1 ).The specific fertilizer dosages were as follows: urea 1.57 g pot −1 , superphosphate 1.41 g pot −1 , potassium sulfate 2.66 g pot −1 .The first topdressing was done at the tillering stage with urea 1.18 g pot −1 .The second topdressing was performed at the booting stage with urea 1.18 g pot −1 and potassium sulfate 2.66 g pot −1 .The water level was maintained at a depth of 3-5 cm after transplanting and continued until two weeks before harvest.Insects, diseases, and grasses were strictly controlled throughout the rice growth period to prevent biomass and yield loss.The indexes were determined at the tillering stage.

Sampling methods and measurements
Dry weight and Na + , K + and Zn 2+ content The dry matter and the levels of Na + , K + , and Zn 2+ in rice leaves were measured in rice plants that had uniform growth at the peak of tillering.For each treatment, 9 samples were taken from both above-ground and underground parts of the plants.These samples were dried at 105 ℃ for 0.5 h and then further dried at 80 ℃ until a constant weight was achieved.The dried leaf samples were ground into a fine powder and sifted.To determine the levels of Na + and K + in rice leaves, 0.500 g of accurately weighed sifted samples were digested using H 2 SO 4 -H 2 O 2 .The levels of Na + and K + in the plants were determined using a flame photometer (FP640, Shanghai Precision Technology Instrument Co., LTD.).To determine the content of Zn 2+ , a 0.1 g sample was accurately weighed using a 2 mm sieve.The weighed sample was then mixed with 3 mL of HNO 3 and 1 mL of HF.Deboiling was carried out using a microwave digestion instrument (MD20H model, Chengdu Opole Instrument Co., LTD., China).After the sample became clear and transparent, the acid was removed using an electric graphite acid catcher (GD25/ GD40, Chengdu Opole Instrument Co., LTD., China).The resulting solution was diluted to a constant volume  www.nature.com/scientificreports/ of 100 mL using ultra-pure water.The content of Zn 2+ in the plants was determined using an inductively coupled plasma emission spectrometer (ICP-1000II, Beijing Hao Micro Technology Co., LTD., China).

Content of malondialdehyde (MDA) and relative electrolyte leakage rate (REL) in rice leaves
The content of malondialdehyde (MDA) and relative electrolyte leakage rate (REL) in rice leaves were measured in rice plants at the peak of tillering.Malondialdehyde content was determined using thiobarbituric acid staining 30 , while REL in rice leaves was determined by Dionisiosese and Tobita (1998) 31 .To measure REL, fresh leaves (1.000 g) were washed with deionized water and placed in a test tube containing 15 ml of deionized water.After incubating for 2 h at room temperature (25 ℃), the conductivity (E1) was measured using a conductivity meter (DS-307, Shanghai Reitz, China).The test tube was then heated to 100 ℃ for 30 min and cooled to room temperature (25 ℃) to measure the electrical conductivity (E2).The relative electrolyte leakage rate (REL) was calculated using the following formula:

Determination of chlorophyll content
At the peak of tillering, rice plants with uniform growth from each treatment were selected to determine the content of leaf pigment.Each treatment consisted of 9 holes.The first fully unfolded leaf from each hole of rice was taken, weighing 0.5 g, and mixed with ethanol, acetone, and water at a ratio of 4.5:4.5:1.The resulting mixture was then standardized to a volume of 25 ml.Absorbance values at 470, 645, and 663 nm were measured using an ultraviolet spectrophotometer (UV-2600, Shimadzu, Japan) to determine the pigment content.The calculation formula used is:

Determination of gas exchange parameters
At the peak of tillering, the net photosynthetic rate (Pn), stomatal conductance (Gs), and intercellular carbon dioxide concentration of the first fully developed rice leaf were measured using the Li-6400, a portable    www.nature.com/scientificreports/photosynthesis measurement system by Li-Cor.The measurements were taken from 9:10 to 11:30 am during a sunny and windless tilling period.Each treatment was repeated 10 times to ensure accuracy.The leaf chamber temperature was maintained at approximately 26 ℃, and the light intensity was set at 800 μmol•m -2 •s -1 .Throughout the measurement process, the CO 2 concentration was kept at 400 μmol•mol -1 , and the relative humidity ranged between 60 and 70%.

Chlorophyll a fluorescence transient and 820 nm reflection
The chlorophyll fluorescence of rice with uniform growth in each treatment was measured at the peak of tillering, with 9 replicates per treatment.The leaves, which had uniformly grown, were dark adapted for 30 min in each treatment.The chlorophyll fluorescence fast induction kinetic curve (OJIP curve) were determined using M-PEA (Hansatech, UK).The OJIP curve was generated under 5000 μmol m −2 •s −1 red light, with a measurement time of 2 s.The initial recording rate was 105 data per second 32 .The O point represents the fluorescence intensity at 0.02 ms, the K point represents the fluorescence intensity at 0.3 ms, the J point represents the fluorescence intensity at 2 ms, and the P point represents the maximum fluorescence, which is generally F p ≈ F m .The OJIP curve is normalized to the OP point, determined by jip test analysis.The normalization is done using the formula , and the parameters and their meanings are shown in Table 3.

Statistical analyses
All data were collected and analyzed using Microsoft Excel 2019 software.Subsequently, the data were analyzed using the SPSS statistical package version 22 (IBM Corp., Armonk, NY, USA).Descriptive statistics were employed to test the mean value and standard error of measurement parameters.One-way analysis of variance (ANOVA) was conducted in this study, and Duncan's multiple comparison method was utilized.The observed differences in comparisons were found to be statistically significant (p < 0.05).The results are presented as standard error (SE).The charts were generated using Origin 2021 software (https:// www.origi nlab.com/ 2021).

Ethics approval and consent to participate
All methods were performed in accordance with the relevant guidelines and regulations.We have obtained permission to collect plant material and seedlings.
Table 3. Formulae and explanation the technical data of the OJIP curves and the selected JIP-test parameters used in this study.
Technical fluorescence parameters Relative variable fluorescence at time t Relative variable fluorescence at the J-step Relative variable fluorescence at the K-step to the amplitude F J -F o Quantum efficiencies or flux ratios φPo = TR o /ABS = 1-F o /F m Maximum quantum yield for primary photochemistry Probability that an electron moves further than Quantum yield for electron transport (ET) Quantum yield for reduction of the end electron acceptors at the PSI acceptor side (RE) Probability that an electron is transported from the reduced intersystem electron acceptors to the final electron acceptors of PSI (RE) Specific energy fluxes and performance indexes www.nature.com/scientificreports/

Plant growth
The effects of ZnSO 4 and ZnO NPs on rice biomass are presented in Fig. 3A.The results indicate that both ZnSO 4 and ZnO NPs significantly influenced the aboveground and root biomass of rice, which were significantly higher than the control group (CK).Moreover, the ZnO NPs treatment exhibited the most favorable results.Figure 3B shows the plant height of rice in saline-sodic land under the three treatments.The results demonstrate that both ZnSO 4 treatment and ZnO NPs are beneficial for the growth of rice in saline-sodic land, with ZnO NPs showing a superior effect.These findings suggest that zinc can alleviate saline-sodic stress and promote the growth of rice in such soil, with ZnO NPs treatment being more effective in this regard.
The concentration of Na + , K + , Zn 2+ , MDA and the relative electrical leakage in rice leaves The effects of ZnSO 4 and ZnO NPs treatments on the concentrations of Na + , K + , and the Na + /K + ratio in rice leaves in saline-sodic soil are shown in Fig. 4A,B.The results indicate that both ZnSO 4 and ZnO NPs treatments led to a decrease in Na + concentration (Fig. 4A) and the Na + /K + ratio (Fig. 4B), while significantly increasing the K + concentration (Fig. 4B) in the leaves.Among the treatments, ZnO NPs had the most pronounced effect on the Na + and K + concentrations and the Na + /K + ratio in rice leaves from saline-sodic soil.Compared to the control (CK) and ZnSO 4 treatments, the ZnO NPs treatment resulted in a significant decrease of 30.9 and 14.1% in the ratio of Na + and Na + /K + in rice leaves, respectively, and a significant increase of 91.6 and 26.2% in the concentration of K + .Figure 4C illustrates the variation in zinc content in rice leaves grown in saline-sodic soil.The findings indicate that the application of exogenous zinc can notably enhance the zinc content in rice leaves.Moreover, the impact of ZnO NPs treatment on zinc content in rice leaves is significantly greater compared to ZnSO 4 treatment.In comparison to the control group (CK) and ZnSO 4 treatment, the ZnO NPs treatment resulted in a substantial increase of 19.6% and 6.6% in the zinc content of rice leaves, respectively.
Figure 4D,E illustrates the changes in malondialdehyde (MDA) content and relative electrolyte leakage rate (REL) in rice leaves from saline-sodic soil.The findings demonstrate that the application of exogenous zinc significantly reduces the MDA content and REL of rice leaves.Moreover, the effects of ZnO NPs treatment on MDA content and REL of rice leaves are even more pronounced.In comparison to the CK and ZnSO 4 treatments, MDA content and REL are notably decreased by 18.4 and 9.2%, and 9.6 and 4.6%, respectively.
Figure 4F presents the correlation between leaf zinc content and malondialdehyde (MDA) content in rice leaves.The results indicate a negative correlation between zinc content in rice leaves from saline-sodic soil and MDA content in rice leaves (P < 0.01), with correlation coefficients of 0.74, respectively.These findings suggest a close relationship between the zinc content of rice leaves from saline-sodic soil and the content of malondialdehyde (MDA) in rice leaves.

Gas exchange parameters
According to Fig. 6, when exposed to saline-sodic stress, exogenous zinc supplementation has been found to alleviate the stress and significantly enhance various parameters related to rice leaf photosynthesis.

The O-P phase, V J , and V I values
Figure 7A,B illustrates the chlorophyll fluorescence induction kinetics (OJIP) curve of rice.When ZnSO 4 and ZnO NPs were applied under saline-sodic stress, the J and I points on the OJIP curve of rice leaves decreased compared to the control (CT).Figure 7B shows that the decline in ∆V t was the largest under ZnO NPs treatment.And, under ZnO NPs treatment, ∆V t had the lowest value at J and I.The amplitude of phase I-P in the W OI ≥ 1 reflects the size of the end electron acceptor pool on the PSI acceptor side, with a smaller amplitude indicating a smaller end electron acceptor pool on the PSI acceptor side.The O-I (Fig. 7D,E) phase analysis revealed that the application of ZnO NPs under saline-sodic stress had the most significant effect on improving the terminal electron acceptor pool on the PSI acceptor side of rice leaves.Additionally, we calculated V J and V I values (Fig. 7C,F), and the results indicated that compared to CT (control treatment), the V J and V I values of rice leaves significantly decreased when ZnSO 4 and ZnO NPs were applied under saline-sodic stress.Specifically, the values of V J and V I were reduced by 24.8% and 9.9% respectively when treated with ZnO NPs, compared to CT and ZnSO 4 treatment which showed reductions of 7.5 and 5.1% respectively.Therefore, under saline-sodic stress conditions, the application of ZnO NPs not only benefits the performance of PSII on the donor side (K-step www.nature.com/scientificreports/reduction) and acceptor side (J-step reduction), but also effectively enhances the size of the electron acceptor pool at the PSI acceptor end.

The PI ABS and specific energy flux for each PSII active reaction center
The results in Fig. 9F demonstrate that the application of ZnSO 4 and ZnO NPs significantly increased the PI ABS index of rice leaves under saline-sodic stress.Additionally, the study revealed that ZnSO 4 and ZnO NPs application reduced the ABS/RC and DI o /RC values (Fig. 9A,C), while increasing the ET o /RC and RE o /RC values (Fig. 9D,E) under saline-sodic stress, and it has no significant impact on TRo/RC (Fig. 9B).Notably, ZnO NPs exhibited the most favorable effect.These findings indicate that, under saline-sodic stress, a greater amount of energy is absorbed by the unit reaction center for trapping and heat dissipation, with less energy being transferred downstream.However, the application of ZnO NPs positively influenced the absorption, transfer, and transfer of energy in the reaction center of rice leaves under saline-sodic stress.Figure 10

Discussion
Saline-sodic soil has a significant impact on plant growth and development 33 .This is mainly caused by the high concentration of Na + in the soil, which leads to excessive absorption of Na + by plants and inhibits the absorption of essential nutrient ions such as K + , Zn 2+ , and Fe 3+ .Such disturbance can have negative effects on plant growth, www.nature.com/scientificreports/development, and even survival 34 .Research has shown that the excessive accumulation of Na + in plant tissues disrupts the integrity and function of cell membrane structures.This disruption leads to an imbalance of reactive oxygen species (ROS) and leads to an increase in MDA content, which increases the relative electricity leakage (REL) of plant tissues 35 .In this study, it was observed that the application of zinc promoted the uptake of Zn 2+ in rice, enhancing its ability to resist saline-sodic.Zinc not only reduced the absorption of Na + and increased the absorption of K + , but also inhibited the production of MDA and reduced REL, thereby maintaining the integrity of the cell membrane 36 .As can be seen in Fig. 4F, there was a significant negative correlation between zinc content and malondialdehyde (MDA) content in rice leaves in saline-sodic land.In recent years, the green synthesis of nanoparticles has attracted much attention because of its advantages of low cost, simplicity and environmental protection 37 .This study found that ZnO NPs was more conducive to the absorption of zinc in rice and had a better alleviation effect on saline-sodic stress.This may be due to the fact that nanoparticles can create higher surface area utilization, which allows rice plants to absorb higher tissue Zn content 38 .
Chlorophyll is a vital substance for photosynthesis in plants as it is closely involved in the absorption and conversion of light energy 39 .The findings of this study revealed that a high pH level reduces the availability of important nutrients such as Zn 2+ and Mg 2+ and inhibits chlorophyll synthesis under saline-sodic stress conditions.The combination of saline-sodic stress leads to ion stress and osmotic stress on rice, resulting in the generation of reactive oxygen species that damage the structure of chloroplasts and accelerate the degradation of rice leaf pigments 40 .Rice plants can counteract photoinhibition by adjusting the ratio of leaf chlorophyll to reduce the amount of captured light energy, thereby facilitating growth throughout the entire growth period 4 .In this study, the application of ZnO NPs zinc significantly increased the contents of chlorophyll a and chlorophyll b in rice leaves, resulting in a decrease in the chlorophyll a/b ratio.The chlorophyll a/b ratio serves as an indicator of plant stress resistance and exhibits a negative correlation with photosynthetic efficiency 41 .Hence, the reduced a/b value due to ZnO NPs application suggests that it enhances the ability of rice to alleviate saline-sodic stress.Furthermore, the application of ZnO NPs also significantly enhances the gas exchange parameters of rice leaves, with the variation in Pn (net photosynthetic rate) being consistent with the intercellular CO 2 concentration 42 .This suggests that plant photosynthesis is influenced by stomatal factors.The similarity in the hydration radius of Na + and K + under saline-sodic stress creates intense competition between Na + and K + channels, which inhibits K + absorption 43 .However, the application of ZnO NPs inhibits Na + absorption and promotes K + absorption, thereby improving stomatal conductance in rice leaves, preserving cell membrane integrity, and creating a favorable environment for photosynthesis, ultimately enhancing the gas exchange parameters of rice.
The results indicated that the application of ZnO NPs enhanced the growth and physiological properties of crops experiencing stress.This can be attributed to the high affinity of nanoparticles for absorption by plants, thereby promoting crop growth and development 44 .A study was conducted to investigate the impact of ZnO NPs on chlorophyll fluorescence in rice leaves under saline-sodic stress using rapid chlorophyll fluorescence kinetic technique and jep-text analysis 45 .Chlorophyll, as a key light-absorbing molecule, provides valuable insights into the structure, conformation, and function of photosynthetic devices through its fluorescence 46 .Plant leaves exhibited distinct OJIP transients under continuous light, representing the multiphase curve induced by chlorophyll fluorescence.The chlorophyll fluorescence gradually increased from the initial fluorescence point O to the maximum fluorescence point P, reflecting the fluorescence kinetic process of OJIP 47 .Rapid chlorophyll fluorescence ascent kinetics typically involve multiple stages, namely O (20 ms, RCS all on), J (2 86 ms), I (30 ms), and P (equal to Fm, RCS all off).Monitoring chlorophyll fluorescence is a widely used method to assess the photosynthetic performance of plants.This technique enables the observation of changes in chlorophyll fluorescence, which in turn provides information about the stability and efficiency of thylakoid membranes 47 .Previous studies have demonstrated that under saline-sodic stress, chlorophyll degradation occurs, leading to the disruption of donor/acceptor side performance and reaction center 48 .Consequently, the photochemical activity of PSII is reduced, and the electron transport ability is inhibited, resulting in an overall decrease in PSII performance and a decline in the photosynthetic ability of rice leaves.The results of this study demonstrated that the application of ZnO NPs caused changes in the fluorescence values of each phase on the curve, with the J-point (V J ) being the most affected.This indicates that the performance of the PSII receptor side was impaired, leading to the destruction of the D 1 protein on the receptor side and hindrance in the electron transfer from Q A to Q B .The quantum yield and flux ratios φ Po , φ Eo , ψ Eo , φ Do reflect the allocation of energy in plant absorption, capture, transfer, and heat dissipation 49 .The results of this experiment revealed that under saline-sodic stress, ZnO NPs increased the values of φ Po , φ Eo , ψ Eo , and decreased the value of φ Do in rice leaves.The decrease in photochemical reaction efficiency (φ Po ) under saline-sodic stress inevitably results in increased energy dissipation, including heat, fluorescence, and energy transfer to other systems.The application of ZnO NPs reduces energy dissipation, increases φ Po , and decreases φ Do 50 .This highlights the significance of Zn in the electron transport chain, and an optimal increase in its content enhances the effectiveness of the electron acceptor and facilitates electron transport between PSI and PSII.www.nature.com/scientificreports/ The performance index PI ABS is a measure of the overall state of the plant's photosynthetic apparatus 51 .It effectively indicates the extent of damage to the PS II of plant leaves under saline-sodic stress.Previous studies have demonstrated that the PI ABS of wheat leaves decreases when exposed to saline-sodic stress, primarily due to osmotic stress and ion stress 52 .In this study, the application of ZnO NPs improved the electron transfer efficiency and photochemical activity of PSII, resulting in an overall improvement in the performance of PSII PI ABS in rice leaves and promoting normal photosynthesis.This improvement may be attributed to the reduction of the V J value by ZnO NPs (Fig. 7C).Other studies have indicated that an increase in V J is associated with the damage level of PSII donor and acceptor sides under saline-sodic stress 53 .The reduction of ZnO NPs application improves the damage level of PSII donor and acceptor sides, thereby contributing to the increase in PI ABS 54 .Furthermore, the application of ZnO NPs was found to significantly reduce ABS/RC and increase ETo/RC and REo/RC in rice 55 .This suggests that ZnO NPs application enhances the light harvesting efficiency and transfer efficiency of rice, further supporting the increase in PI ABS .
Saline-sodic stress has been found to have a negative impact on chlorophyll synthesis and the electron transport chain between PSI and PSII 55 .Zinc not only reduces the absorption of Na + by rice, thereby alleviating salinesodic stress, but also aids in the synthesis of chlorophyll.This creates an optimal environment for photosynthetic electron transfer, effectively promoting electron transfer between PSI and PSII 56 .Compared to zinc sulfate, ZnO NPs demonstrate a more favorable effect.ZnO NPs are widely utilized in various fields.The unique nanostructure and nanoproperties of ZnO NPs have garnered significant attention from scientists 38 .In the realm of agricultural production, these nanoparticles have demonstrated promising effects, including the promotion of seed germination and seedling growth, mitigation of abiotic stress, and enhancement of plant resistance.Nevertheless, it  2.The mean ± SE (n = 9) of nine replicates was used, *: different letters were statistically significant at the p < 0.05 level.CT: Saline-sodic soil, Z: saline-sodic soil applied zinc sulfate, nZ: saline-sodic soil applied nano-zinc oxide.
is crucial to acknowledge the potential negative impacts of ZnO NPs, as several studies have indicated a dosedependent effect 57 .High doses of ZnO NPs can hinder plant growth by inhibiting germination and chlorophyll biosynthesis, leading to reduced biomass accumulation and ultimately affecting crop development 58 .Therefore, the application of ZnO NPs should be carefully managed to ensure optimal utilization.

Conclusion
Compared to ZnSO 4 , the use of ZnO NPs has been found to be more beneficial in enhancing the growth and physiological characteristics of rice in saline-sodic environments.The application of ZnO NPs not only hinders the absorption of Na, thereby reducing the detrimental effects of saline-sodic stress on rice, but also facilitates the absorption of Zn, leading to an increase in the chloroplast pigment content of rice under saline-sodic stress and promoting efficient photosynthesis.Additionally, the application of ZnO NPs enhances the performance of the electron acceptor side of the electron transport chain, thereby facilitating electron transfer between PSII and PSI.In conclusion, this study emphasizes the significance of zinc oxide nanoparticles in improving the growth and photosynthetic efficiency of rice in saline-sodic soils.

Figure 1 .
Figure 1.The pot experiment was conducted at Jilin Agricultural University in Jilin Province, China.The map is done using arcgis 10.8 (https:// www.arcgis.com/ index.html).

Figure 2 .
Figure 2. TEM scanning of ZnO NPs particles in deionized water.

Figure 5 Figure 3 .
Figure5illustrates the changes in pigment content in rice leaves grown in saline-sodic soil.The results indicate that the application of exogenous zinc can effectively enhance the pigment content in rice leaves when subjected to saline-sodic stress.Furthermore, the treatment with ZnO NPs has a more pronounced effect on the levels of chlorophyll a, chlorophyll b, chlorophyll a + b, and carotenoids.Compared to the control group (CK), the ZnO NPs treatment significantly increased the content of chlorophyll a, chlorophyll b, chlorophyll a + b, and carotenoids in rice leaves by 52.1, 60.5, 54.6, and 22.7% respectively.Additionally, the application of ZnO NPs led to a significant decrease in the chlorophyll a/b value.Compared to the CK and ZnSO 4 treatment, the ZnO

Figure 8
Figure8presents the effective parameters, such as the quantum yield of rice seedlings, under saline-sodic stress.The application of ZnSO 4 and ZnO NPs significantly increased the values of φ Po , ψ Eo , φ Eo, φ Ro and δ Ro in rice leaves, while the values of φ Do decreased significantly.Notably, the treatment with ZnO NPs showed the most significant improvement.Compared to the control group (CT), the values of φ Po , ψ Eo , and φ Ro increased by 30.7, 49.8, and 61.8%, respectively, while the value of φ Do decreased by 41.2%.These findings suggest that the use of ZnO NPs is more advantageous in enhancing the quantum yield and efficiency of rice under saline-sodic stress.It also reduces the energy dissipation ratio and improves the photosynthetic fluorescence performance index.

Figure 5 .
Figure 5.The effects of ZnO NPs on chlorophyll a (A), chlorophyll b (B), chlorophyll a + b (C), chlorophyll a/b (c) and carotenoid (D) contents in rice leaves under saline-sodic stress.The mean ± SE (n = 9) of nine replicates was used, *: different letters were statistically significant at the p < 0.05 level.CT: Saline-sodic soil, Z: saline-sodic soil applied zinc sulfate, nZ: saline-sodic soil applied nano-zinc oxide.

Figure 6 .
Figure 6.The effects of ZnO NPs on net photosynthetic rate (A), stomatal conductance (B), intercellular carbon dioxide concentration (C) and transpiration rate (D) in rice leaves under saline-sodic stress.The mean ± SE (n = 9) of nine replicates was used, *: different letters were statistically significant at the p < 0.05 level.CT: Saline-sodic soil, Z: saline-sodic soil applied zinc sulfate, nZ: saline-sodic soil applied nano-zinc oxide.

Figure 7 .
Figure 7.The effects of ZnO NPs on the relative variable chlorophyll fluorescence V t and O-I phases of rice under saline-sodic stress.(A, D) shows the relatively variable V t and O-I phases of chlorophyll fluorescence in rice leaves.B, E showed relatively variable chlorophyll fluorescence ∆V t and W OI ≥ 1 in rice leaves.(C, F) V J and V I are relatively variable fluorescence of step J and step I.The mean ± SE (n = 9) of nine replicates was used, *: different letters were statistically significant at the p < 0.05 level.CT: Saline-sodic soil, Z: saline-sodic soil applied zinc sulfate, nZ: saline-sodic soil applied nano-zinc oxide.

Table 1 .
Basic physical and chemical properties of the tested soil.

Table 2 .
Application amounts of ZnSO 4 and ZnO NPs.

ZnSO 4 ZnO NPs
These parameters include the net photosynthetic rate, stomatal conductance, intercellular carbon dioxide concentration, and transpiration rate.Compared to the control group (CK), net photosynthetic rate of rice leaves treated with ZnSO 4 and ZnO NPs increased by 19.1 and 30.5%, stomatal conductance increased by 38.7 and 46.8%, transpiration rate increased by 20.1 and 28.1%, and intercellular carbon dioxide concentration increased by 31.5 and 67.1%, respectively.Thus, zinc supplementation, particularly in the form of ZnO NPs, can effectively enhance the photosynthesis of rice leaves in saline-sodic soil.